Why Digital Cameras Have Mechanical Shutters

Background

Ever wondered why digital
cameras, particularly high-end digital SLR's, have mechanical shutters?
The sensor is electronic, so why can't it be told to simply sample the
light for the length of time specified by the shutter speed? Why
can't the sensor just start accumulating light (what is sometimes
referred to as a "charge"), wait a specified length of time, and then
stop accumulating light at the end of the exposure time? Let's
take a quick look at the reason mechanical shutters are used in digital
cameras.

The Shutter Itself

Digital cameras use several different types of mechanical shutters, but
all of them serve the same purpose. They block light from reaching
the sensor when closed and move out of the way to let light accumulate
on the sensor while open. Of course, the first thing that comes to
mind is that the sensor, being an electronic device, should be able to
simply turn on/off electronically. Why is the shutter even needed?
Well, in fact, many cameras do use an electronic shutter that simply
turns on/off the "light reading" capability of the sensor when needed.
Many pocket point-and-shoot cameras use this technique. Pocket
cameras that use the rear LCD to preview the picture are sometimes set
up this way and hence have no mechanical shutter at all.
Realizing that some cameras have all-electronic shutters while others
have mechanical shutters, it's obvious that there are pros and cons to
both designs.

Sensor Types

Interline Transfer

Cameras, typically smaller point-and-shoot cameras, that use no
mechanical shutters typically use an interline transfer sensor. An
interline transfer sensor dedicates a portion of each pixel to store the
charge for that pixel. The added electronics necessary to be able
to store the charge for each pixel reduces the fill factor of the pixel,
in turn reducing it's ability to capture light since a portion of each
pixel is not light sensitive. Microlenses can be used to
compensate but they are not 100% efficient and they can add expense to
the design. Interline transfer sensor's typically have higher
noise levels and lower sensitivity than the full frame sensor's used in
high end digital SLR's. One obvious benefit is that this design
eliminates the need for a potentially bulky mechanical shutter and can
turn a purse size camera into a shirt pocket camera.

Full Frame

Digital cameras that use a mechanical shutter typically use a type of
sensor called a full frame sensor. Unlike the interline transfer
sensor (above), the full frame sensor has no circuitry on the pixel to
store the charge that builds up as light contacts the array.
Cameras that use a mechanical shutter typically bleed off any residual
electrical charge while the shutter is closed, open the shutter, and
then close the shutter. Once the mechanical shutter is closed,
circuitry is then used to shift the charge from each pixel into a
storage area. Since the pixels on the sensor remain "live" during
readout, if the shutter remained open, light would continue to alter the
charge accumulated by each pixel during the shifting operation which
could result in blur or ghosting.

Mechanical shutters: the
bottom line

In layman's terms, a mechanical shutter is used to control how long the
pixels on an image sensor collect light. A simple mechanical
shutter can be used to turn the entire sensor array on/off during the
exposure. This eliminates the need for added electronics at each
pixel location that would be used to turn on/off the pixel and store the
charge (accumulated light). By using a mechanical shutter, a
simpler, less expensive, and more efficient sensor can be used: one that
has a higher fill factor (uses more of each pixel to actually capture
light). Of course, nothing is ever cut and dried. Some
cameras use both a mechanical and an electronic shutter! In these
cases, the electronic shutter is used to supplement the mechanical
shutter by providing features like a faster flash sync speed where
mechanical shutters are just not fast/accurate enough. Most
digital SLR cameras that use a mechanical shutter, however, use the
mechanical shutter to control the amount of charge accumulated on the
sensor as this simple mechanical device can be used to simplify the
circuitry on the sensor itself thereby generally improving image quality
and reducing noise.

Summary

This article is designed to answer
the question of why a digital camera, admittedly a "solid state" device
that shouldn't logically need any moving parts other than a focus
mechanism would need a mechanical shutter. The answer, on the
surface, turns out to be relatively simple and I hope I've answered the
question so that most people can grasp the concept.